Fig. 1. Piezoresistive designs for engineering electronic skin. (a) Developmental history of flexible skin-like electronics (e.g. milestones in the shear force measurement, large-area manufacturing, spatial mapping, bioinspired manufacturing, interconnection between e-skin and live neurons, and self-healing capability)^10-17 . (b) Scheme illustrating the design and applications of piezoresistive sensors.

Fig. 2. Principles of piezoresistive effect. (a) Piezoresistance based on the geometric changes of metals and conducting polymers. i and ii represent the block and planar material geometries, respectively^41,47. (b) Piezoresistance of semiconductor. Scheme illustrating the changes of charge carrier and energy band upon traction along the [111] direction in p–Si⁴⁷. (c) Piezoresistance of composite materials based on the changes of conducting filler concentration and inter-filler distance⁴⁸. (d) Structural piezoresistance based on the contact area and point changes of conducting architecture. Figure reproduced with permission from: (b) ref.⁴⁷, Elsevier.

Fig. 3. Development of piezoresistive sensors based on various materials. (a) Fabrication scheme and optical images of a hybrid metallic foam⁴⁶. (b) Schematic illustrating the fabrication of CNTs/FKM nanocomposite⁵⁰. FKM, fluoroelastomer; CNTs, carbon nanotubes; and TAIC, 1,3,5-triallyl-1,3,5-triazine-2,4,6 (1H,3H,5H)-trione. (c) Scheme illustrating the assembly of a MG/PU piezoresistive sensor⁵¹. MG, modified-graphite; PU, polyurethane; and PDMS, polydimethylsiloxane. (d) A flexible tactile sensor assembled from the 8x8 array of Si-strain gauges and the Si thin film transistors⁶⁴. (e) Scheme illustrating the sandwich-structure of a PEDOT:PSS/Ag NW/PDMS component film⁷⁴. PEDOT:PSS, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate); and Ag NWs: Ag nanowires. (f) mGN fillers form new conducting paths under pressure⁸⁰. mGN, magnetic reduced graphene oxide@nickel nanowire. Figure reproduced with permission from: (a) ref.⁴⁶, (f) ref.⁸⁰, American Chemical Society; (c) ref.⁵¹, under a Creative Commons Attribution 4.0 International License; (d) ref.⁶⁴, AIP Publishing.

Fig. 4. Piezoresistive design and manufacture with singular structure. (a) Piezoresistance of an artificial structured sensor based on contact area changes⁹¹. ITO, indium tin oxide; PET, polyethylene terephthalate; SWCNTs, single-walled carbon nanotube; PDMS, polydimethylsiloxane. (b) Scheme illustrating the lotus-leaf-inspired piezoresistive design and assembly⁹⁸. (c) Scheme illustrating the rose-petal-inspired piezoresistive design and assembly⁹⁹ and the shark-skin-inspired piezoresistive design and assembly¹⁰¹. (d) Scheme illustrating the spider-leg-joint-inspired piezoresistive design with plenty of cracks¹⁰². Figure reproduced with permission from: (a) ref.⁹¹, (d) ref.¹⁰², Elsevier; (c) ref.¹⁰¹, Elsevier and ref.⁹⁹, Royal Society of Chemistry.

Fig. 5. Sensitivities of different microstructure sensors. ITO, indium tin oxide; MWNT, multiwalled carbon nanotubes; and PDMS, polydimethylsiloxane. Figure reproduced with permission from ref.⁹⁴, under a Creative Commons Attribution 4.0 International License.

Fig. 6. Piezoresistive design and manufacture with hierarchical structure. (a) Multiple dome structures as piezoresistance sensing layer⁸⁶. (b) Multiple structure comprising of dense protuberances and porous structure as piezoresistance sensing layer¹⁰⁶. HPM, hybrid porous microstructure; and CNT, carbon nanotubes. (c) Piezoresistive sensitivities based on the sensing layers comprising of rough-to-rough, rough-to-flat and flat-to-rough surfaces¹⁰⁷. Figure reproduced with permission from: (a) ref.⁸⁶, (b) ref.¹⁰⁶, (c) ref.¹⁰⁷, American Chemical Society.

Fig. 7. Design and manufacture of fibre piezoresistive sensor. (a) Schematic of a rGO-Ag NW@cotton fibre, through immersion of cotton fibres in a reductive solution containing GO and AgNW¹¹². GO, graphene oxide; rGO, reduced GO; and AgNW, Ag nanowires. (b) Scheme of an electrospun fibre piezoresistive sensor and its sensing mechanisms during pressure and blending¹¹⁰. KL, kraft lignin. (c) Scheme of a wet-spinning single-fibre piezoresistive sensor¹²². Figure reproduced with permission from: (a) ref.¹¹⁰, (b) ref.¹¹², Elsevier; (c) ref.¹²², American Chemical Society.

Fig. 8. Piezoresistive design and manufacture with spongy structure. (a) Freeze drying for the fabrication of piezoresistive sponge⁶⁸. (b) Directional freeze drying of the fabrication of piezoresistive wave-shaped sensing layers¹²⁷. CNC, cellulose nanocrystals. (c) Dip coating of as-fabricated sponge with conducting materials¹³⁰. PU, polyurethane; and MWCNT, multiwalled carbon nanotubes. (d) Sacrificial template for the fabrication of piezoresistive sponge¹³¹. (e) Sponge-based hierarchical structure (e.g. cracks) for piezoresistive sensor¹³⁴. Figure reproduced with permission from: (a) ref.⁶⁸, (d) ref.¹³¹, (e) ref.¹³⁴, American Chemical Society; (b) ref.¹²⁷, (c) ref.¹³⁰, Royal Society of Chemistry.

Fig. 9. Additive manufacturing for piezoresistive sensor. (a) Drop-on-demand material jetting for the facial fabrication of piezoresistive structures¹⁴³. MCF, milled carbon fibers; and SR, silicone rubber. (b) 3D printing of human-skin-inspired texture as piezoresistive sensing layers¹⁴⁴. CNT, carbon nanotube. (c) Application of a 3D printed piezoresistive sensor for robotic fingertips to sense force¹⁴⁵. (d) 3D printing of conducting composite for piezoresistive sensor¹⁴⁸. SIS, polystyrene–polyisoprene–polystyrene. (e, f) 3D printing of different structure parameters (e.g. diameter, interaxial angle, and interlayer space) for piezoresistive sensor¹⁴⁹. GF, gauge factor. Figure reproduced with permission from: (a) ref.¹⁴³, (e, f) ref.¹⁴⁹, Elsevier; (c) ref.¹⁴⁵, American Chemical Society; (d) ref.¹⁴⁸, Royal Society of Chemistry.

Fig. 10. Piezoresistive sensor for health monitoring. (a) Detection of physical activities by piezoresistive sensor, including the swallowing (A and B: the pharyngeal and esophageal phase, respectively) and the posture of human back¹⁵². (b) Detection of physiological activities (e.g. wrist and jugular venous pulse) at high sensitivities¹⁵³. (c) Detection of myocardic activities (e.g. heartbeats at breathing and not breathing) by a piezoresistive sensor¹⁵⁴. (d) Identification of different mechanical stimuli, including pressure, shear and torsion force¹³². Figure reproduced with permission from: (a) ref.¹⁵², (d) ref.¹³², Elsevier; (b) ref.¹⁵³, (c) ref.¹⁵⁴, American Chemical Society.

Fig. 11. Piezoresistive sensor for intelligent healthcare. (a) Piezoresistance for visiual, alarm, wireless and implanted applications³⁵. (b) Triode-mimicking pressure sensor for intelligent shoe pad¹⁵⁹. (c) Scheme illustrating a monitoring system developed based on the intelligent shoe pad in (b)¹⁵⁹. (d) Continuous and multiple signals from the integrated gait monitoring system in (c)¹⁵⁹. Figure reproduced with permission from: (a) ref.³⁵; (b–d) ref.¹⁵⁹, American Chemical Society.

Fig. 12. Piezoresistive sensor for intelligent speech recognition. (a) Piezoresistive detection of sound (e.g. word recognition, volume detection, and voice recognition)⁶⁸. (b) Response of a MXene-based piezoresistive sensor to the audio outputs at different volumes¹⁶². (c) Anti-interference voice recognition by a skin-attachable piezoresistive sensor¹⁶⁵. Figure reproduced with permission from: (a) ref.⁶⁸, (b) ref.¹⁶², (c) ref.¹⁶⁵, American Chemical Society.

Fig. 13. Piezoresistive sensor for prosthetics and robots. (a) Manipulating the robot arm by a piezoresistive sensor for music playing¹⁶⁹. (b) Large-area force distribution detected by fibre piezoresistive sensor array¹⁷¹. (c) Piezoresistive sensor for the real-time monitoring of robot−tissue collision/interaction in surgical robots¹⁷³. (d) Development of an artificial afferent nerve based on multiple piezoresistive sensors¹⁷⁵. Figure reproduced with permission from: (a) ref.¹⁶⁹, (b) ref.¹⁷¹, (c) ref.¹⁷³, American Chemical Society.

Table 1. Summary on the parameters of current piezoresistive sensors.